IS hostages, strongholds stand between US-backed forces and Raqqa's capture

Members of Syrian Democratic Forces take up positions on the rooftop of a house during a battle with Islamic State militants in Raqqa, Syria October 1, 2017. REUTERS/Erik De Castro TPX IMAGES OF THE DAY

Smoke rises in the stadium as members of Syrian Democratic Forces battle with Islamic State militants in Raqqa, Syria October 1, 2017. REUTERS/Erik De Castro

A member of Syrian Democratic Forces who was wounded during a battle with Islamic State militants smokes a cigarette in Raqqa, Syria October 1, 2017. REUTERS/Erik De Castro

A member of Syrian Democratic Forces drives an armoured fighting vehicle past destroyed buildings in Raqqa, Syria September 30, 2017. REUTERS/Erik De Castro

A fighter of Syrian Democratic Forces takes up a position inside a house in Raqqa, Syria October 1, 2017. Picture taken October 1, 2017. REUTERS/Erik De Castro

A view of Raqqa's National Hospital, last stronghold of the Islamic State militants, in Raqqa, Syria September 30, 2017. REUTERS/Erik De Castro TPX IMAGES OF THE DAY

Teen fighters of Syrian Democratic Forces joke each other in Raqqa, Syria October 1, 2017. Picture taken October 1, 2017. REUTERS/Erik De Castro

Fighters of Syrian Democratic Forces hold a meeting inside a house that has become their camp in Raqqa, Syria October 1, 2017. Picture taken October 1, 2017. REUTERS/Erik De Castro

Members of Syrian Democratic Forces take up positions inside a building as they battle Islamic State militants in Raqqa, Syria October 1, 2017. REUTERS/Erik De Castro

Members of Syrian Democratic Forces transport a suicide car bomb used by the Islamic State militants in Raqqa, Syria October 1, 2017. REUTERS/Erik De Castro

A view of destroyed vehicles and buildings in Raqqa's old city, in Syria October 1, 2017. Picture taken October 1, 2017. REUTERS/Erik De Castro

Smoke rises at the positions of Islamic State militants after an air strike carried out by Syrian Democratic Forces in Raqqa, Syria September 30, 2017. REUTERS/Erik De Castro

Fighters of Syrian Democratic Forces take up positions from the roof top of a house in Raqqa, Syria October 1, 2017. Picture taken October 1, 2017. REUTERS/Erik De Castro

A member of Syrian Democratic Forces rests inside a house that has become their makeshift camp, in Raqqa, Syria September 30, 2017. REUTERS/Erik De Castro

A member of Syrian Democratic Forces walks along a street in the Old City of Raqqa, Syria October 1, 2017. REUTERS/Erik De Castro

A view of buildings destroyed during clashes between Syrian Democratic Forces and Islamic State militants in Raqqa, Syria September 30, 2017. REUTERS/Erik De Castro

A member of Syrian Democratic Forces prays on his position during a battle with Islamic state militants in Raqqa, Syria October 1, 2017. REUTERS/Erik De Castro

A car drives past buildings destroyed during clashes between Syrian Democratic Forces and Islamic State militants in Raqqa, Syria September 30, 2017. REUTERS/Erik De Castro

A fighter of Syrian Democratic Forces who was slightly wounded from battle with Islamic State militants rests inside their camp in Raqqa, Syria October 1, 2017. Picture taken October 1, 2017. REUTERS/Erik De Castro

Beyond it, a roundabout where the jihadists once displayed the heads of their enemies, crucified people and held military parades at the height of their expansion is another strategic prize sought by the U.S.-backed militia alliance.

Commanders directing the battle on the ground say seizing these and a nearby stadium, Islamic State's last strongholds in the city, could take as little as a week once a final assault begins against just a few hundred remaining militants.

But the ultra-hardline group is holding civilian hostages in the hospital and stadium and using sniper fire, booby traps and tunnels that emerge behind SDF lines to slow the battle.

The SDF faces a tough final showdown with IS which commanders say will end at the hospital, now almost completely surrounded.

"There are many civilians being held. We can't use heavy weaponry or air strikes around the hospital or stadium, so we'll encircle them as we advance," commander Haval Gabar said at the front-line base, a home that SDF units captured last week.

"The hospital will be the last point (in Raqqa) to be freed," he said on Saturday, as bullets coming from the sprawling medical complex whizzed over the base.

The SDF, an alliance of Kurdish and Arab militias dominated by the Kurdish YPG, has been fighting since June to drive Islamic State from Raqqa city, backed by air strikes and special forces from a U.S.-led coalition.

"THERE'S NO ADVANCING"

The assault, which YPG officials initially predicted would take weeks, has dragged on as Islamic State bogs down forces with tactics used in other bastions such as Iraq's Mosul. Senior Kurdish commanders recently said Raqqa would fall by the end of October.

"Right now there's no advancing," Gabar, 25, said.

"There have been many attacks from behind us" with militants launching surprise raids from a network of tunnels they dug after marauding through swathes of Syria and Iraq and capturing Raqqa in 2014, he said.

"When that happens we divert forces from front-line assaults to deal with the infiltration. But it doesn't take long, maybe half an hour to deal with each attack."

The home the SDF was using as a base had an IS tunnel emerging into its living room - now blocked up with furniture.

Commander Zilzal Tarhams said militants emerged from a second nearby tunnel two days before and fired a rocket into a house occupied by the SDF.

"If we find tunnels we usually cave them in with explosives. There are so many," he said.

The commanders were confident of total victory soon, however, with Islamic State surrounded in a small portion of the city which houses the remaining strategic sites.

Gabar, who directs fighting on the western front, said forces were waiting for the order for a final push, after which the roundabout, stadium and hospital would quickly fall.

"When we begin it will be quick, maybe 10 days or so," he said. Tarhams predicted a week.

"It's really because of the civilians presence that it's taking us some time," Gabar said. There were thousands of residents still trapped in the city, including an unknown number at the stadium and hospital, he said.

CAPTURE BEFORE WINTER

Air strikes that have been used heavily in other parts of the city are not as frequent around the remaining sites, although missiles still slam into buildings in the city center.

Gabar and Tarhams did not say how the SDF planned to capture the stadium and hospital, both of which provide high vantage points over the city center, while avoiding the deaths of residents held there.

Islamic State leader Abu Bakr al-Baghdadi commanded his militants not to negotiate or surrender in an audio message released recently.

Raqqa's complete capture will end the group's sway in the city from where it plotted deadly attacks abroad, and projected its power in 2014 parading through streets flying black flags on military vehicles.

The two SDF commanders estimated there were no more than 350 militants left in Raqqa and said these were not the elite of the group. Many leaders are believed to have fled to Deir al-Zor where the SDF and Syrian government forces are pressing competing offensives against IS.

Other tactics the jihadists were using to slow SDF advances include laying booby traps, Tarhams said. "They put mines under rugs in homes and use motion sensors to set off bombs. It's a huge obstacle."

Battle weary, the commanders were keen to capture the last strategic sites as soon as possible.